1. Field of the Invention
The present invention relates to a magnetoresistive element and a method for producing the same.
2. Description of the Related Art
Tunnel magnetoresistive (TMR) elements have been the subject of in-depth research for application to magnetic heads, MRAMs and the like since the TMR elements having a magnetic material/tunnel insulating layer (tunnel layer)/a magnetic material as the basic structure were found to achieve high magnetoresistance change ratio (MR ratio).
The TMR elements utilize the fact that the tunneling probability between the magnetic materials is changed with the relative magnetization angle of the two magnetic materials sandwiching the tunnel layer. Aluminum oxide is commonly used as the tunnel layer. In general, aluminum oxide is formed by oxidizing a metal aluminum film formed on a magnetic material. Exceptionally, an example of a TMR element produced with boron nitride (BN) that exhibits a higher MR than that produced with aluminum oxide has been reported (JP4-103013A). However, referring to many other study examples, it is believed that aluminum oxide provides the highest MR at present.
In order to use a TMR element in a magnetic device such as a magnetic head and an MRAM, it is desirable to reduce the element size in order to improve the magnetic recording density and the memory packaging density. The tunnel junction resistance is increased with decreasing element size, so that the smaller junction resistance per unit area is better. One effective method for reducing the junction resistance is to reduce the thickness of the tunnel layer. However, when the thickness of a metal aluminum film increasingly is reduced, aluminum is formed in an island shape, so that the thickness of the tunnel layer becomes increasingly non-uniform, which eventually makes it difficult to form a film.
When the thickness of the tunnel layer is reduced, the MR ratio also is reduced. This seems to occur for the following reason. As the tunnel layer becomes thin, the magnetostatic coupling or tunnel exchange coupling between the magnetic layers via the tunnel layer is increased by a so-called orange-peel effect, so that a preferable relative magnetization angle between the magnetic layers cannot be obtained or leak current is increased.